In mobile devices represented by mobile phones, semiconductor memories such as DRAM, SRAM, and FLASH memory are used. Each of the memories has advantages and disadvantages, and currently, they are subjected to use depending on the characteristics thereof.
For example, while DRAM has a large capacity, the access speed thereof is low. On the other hand, while the speed of SRAM is high, since many transistors as much as 4 to 6 transistors are required per one cell, it is difficult to increase the degree of integration, and it is not suitable for a large-capacity memory.
Moreover, DRAM and SRAM always require power supply to retain data (volatile). On the other hand, while FLASH memory does not require power supply for electrical memory retention since it is non-volatile, it has disadvantages that the number of times of rewrite or erase is limited to about 105 times and that the rewriting speed is slower than other memories by several digits.
If a universal memory having advantages of the DRAM, SRAM, and FLASH memory in combination can be realized, a plurality of memories can be integrated in one chip, and downsizing and function enhancement of mobile phones or various mobile devices can be achieved. Furthermore, if all semiconductor memories can be replaced, the impact is significantly large. Factors required for the universal memory include, for example, increasing the degree of integration (increasing capacity) to the level of DRAM, high-speed access (write/read) to the level of SRAM, non-volatility like FLASH memory, and low-power consumption that can withstand small battery drive, etc.
Among next-generation non-volatile memories called universal memories, what is currently attracting attention the most is a phase-change memory. The phase-change memory uses a chalcogenide material that is used in optical disks such as CD-RW and DVD and similarly stores data by the difference between a crystalline state and an amorphous state. The difference between the phase-change memory and the optical disk resides in the writing/reading method. The optical disk utilizes transmission and reflection of light typified by laser; on the other hand, in the phase-change memory, write is performed by Joule heat generated by a current, and a signal is read by the difference of resistance values caused by phase change.
Regarding a phase-change memory cell, there is a description in Technical Digest of International Electron Device Meeting, 2001, p. 803-806 (Non-Patent Document 1). Regarding the phase change of chalcogenide materials, there is a description in Journal of Applied Physics, Volume 87, Issue 9, May 2000, p. 4130 (Non-Patent Document 2).
Moreover, Japanese Patent Application Laid-Open Publication No. 2003-174144 (Patent Document 1), U.S. Patent Application Publication No. US 2004/0026731 (Patent Document 2), and U.S. Patent Application Publication No. US 2003/0047727 (Patent Document 3) describe techniques for inserting an adhesive layer between an electrode and a chalcogenide material layer for preventing exfoliation between the electrode and the chalcogenide material layer in a phase-change memory.
In addition, as a result of prior art document research carried out by the inventor of the present invention, Japanese Patent Application Laid-Open Publication No. 2004-288843 (Patent Document 4) describes a structure in which a stacked film comprising an amorphous thin film containing chalcogenide and a top electrode is formed so that the stacked film is connected to a bottom electrode via an opening formed in an insulating film on the bottom electrode formed on a silicon substrate.    Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2003-174144    Patent Document 2: U.S. Patent Application Publication No. US 2004/0026731    Patent Document 3: U.S. Patent Application Publication No. US 2003/0047727    Patent Document 4: Japanese Patent Application Laid-Open Publication No. 2004-288843    Non-Patent Document 1: Technical Digest of International Electron Device Meeting, 2001, p. 803-806    Non-Patent Document 2: Journal of Applied Physics, Volume 87, Issue 9, May 2000, p. 4130